What does "conversation timed out" mean in an email bounce?

Unfortunately there's no standard here ... the failure could be for many
different reasons.

The best I can offer here are some guesses as to what might be happening
given the clues at hand.

First let's explain that error:

timed out
while sending RCPT TO

When mail servers talk to each other using the SMTP protocol, or even when
your email program sends mail using SMTP to a server, the machines involved
begin what can best be described as a "conversation". The machine sending email
sends a command, and the machine that would be accepting the message responds
to each command. It's a very simple back and forth conversation, each machine
responding to the other in turn until the message has been sent.

A "time out" is just one machine's failure to respond when it's supposed to.
Machine one says something as part of the conversation and machine 2 responds
with ... nothing. After a while machine 1 just gives up and reports the time
out ("I waited too long and didn't get an answer") error.

Now, the RCPT command is the first command in the conversation that actually
identifies who the mail is going to; it includes the email address. A message
with several recipients will include several RCPT commands.

So what the error you're seeing indicates that the sending machine has said
"this email is for so-and-so", and the accepting machine stopped responding.

Great. What does that mean?

We don't know. There's no standard for that behavior. However, here
are a few guesses:

• The email address you're sending to is invalid or doesn't exist. This is
  perhaps the most likely scenario of all. In order to thwart or at least slow
  down spammers who will blast email to thousands of addresses whether they exist
  or not many email servers have elected simply to stop responding when an
  invalid email address is detected. The reasoning is simple: the sender would
  get an error one way or another anyway, since the email address is invalid, but
  by forcing the sending server to wait for the time out is that sender happens
  to be a spam-bot, then it's being forcibly slowed in its effort to spam the
  world.

• The email address you're sending to has some other kind of problem. This is
  less likely, but still possible. For example a too-full mailbox or other kind
  of temporary problem could lead to this. It's not supposed to, but it's
  possible.

• Your sending mail server has been identified as a source of spam. Since
  you're indicating that other email addresses to this destination domain work,
  this is also less likely but given the complex nature of spam detection
  algorithms but it is still possible.

• The recipient mail server simply has a problem. Impossible to say exactly
  what kind of problem, but a misconfiguration or other issue could result in
  this type of behavior.

So all that tells you very little.

Here's what I would do in your situation: first, triple check that the email
address you are sending to is correct and really does exist - that's by far the
most likely issue. If that all looks correct, then your only recourse is to
contact the postmaster or person responsible for the receiving server and see
if they can tell you why that single email address might be failing for
you.